The shape of the segmented cold crucible, a side wall of the crucible body of the so-called cold crucible melting, is classified into two types of the first and second types as follows.
1) A first type conventional melting furnace
a) As shown in FIG. 6, a water cooled copper crucible 1 as a furnace body is constructed as having a side wall 1a, a bottom wall 1d and a bottom part 4 placed further under the bottom wall 1d. PA1 b) The crucible body comprises an induction heating coil 2, which is placed surrounding the outside wall face of the aforesaid water-cooled segments 1a' and is supplied by high frequency or intermediate frequency electric power to melt the metal or metals received in the crucible by induction heating without contacting with the crucible inner face, and PA1 c) a supply and a control system for supplying and controlling the above-mentioned melting power and cooling water. PA1 a) Improving the electric magnetic force by increasing the magnetic flux at the crucible bottom, PA1 b) Shutting off the path of thermal conductivity from the molten metal by making two adjacent portions separated, that is, by separating the molten metal and side wall of the crucible, PA1 c) To make the induction heating power at these portion contribute to the heating, thereby to decrease the electric power to maintain the molten state, PA1 d) Increasing the melting power efficiency to decrease the amount of skull.
At the bottom of the side wall 1a of the crucible 1, a bottom flange 1c formed by extending radially and outwardly from the base part of the bottom, a flange 4a corresponding to the flange 1c extending also radially from the top face of the bottom part 4 and they are connected by bolts 7.
While, the side wall 1a of the metallic crucible having a shape of a hollow bottomed cylindrical shape is split into a plurality of segments 1a' by a plurality of slits 1b extending vertically.
Each of the segments 1a has within its interior a double walled tube 5 divided into an inlet and an outlet opening for a cooling water, each of them is connected to an inlet passageway 5a and an outlet passageway 6a and they constitute a segment and a crucible of a water cooled metallic crucible body as an integrated body of these segments 1a'.
To summarize, the conventional first type cold crucible 1 is split into a plurality of segments 1a' by slits 1b, and each of these segments is composed of independently operable crucible side wall 1a having an inlet passage 5a and an outlet passage 6a, a bottom wall 1d, a bottom part 4, induction heating coil disposed around the crucible 1, pipes for cooling water for this coil and a supply and control systems for these power and water systems.
By virtue of this construction, metal or metals to be melted which are supplied into the crucible are induction heated by subjecting to exposure to alternating current supplied to this coil or coils and are melted to a molten metal or alloy in the crucible.
The upper surface of the molten metal, due to the balance of power caused by electric magnetic power acting on the upper surface of the molten metal or alloy and to the static pressure given by the weight of the molten metal or alloy, will be raised upwards being separated from the inside face of the side wall of the crucible and is kept as a dome-like molten metal 3, while the lower bottom of the molten metal 3, the interior of the side wall 1a and between the top face of the bottom wall 1d is kept as a skull 9 as a skin of solidified metal formed by the water-cooled copper crucible.
2) A second type conventional cold crucible melting furnace
Following are features different from the above-mentioned first type cold crucible melting furnace.
As shown in FIGS. 7A and 7B, a plurality of segments 13 form a side wall of a plurality of pairs 13a and 13b of two adjacent ones, the one of which 13a forms an inlet passage 15a and the other of 13b has an outlet passage 15b which is communicated with outlet passage 15a at the top, and these pair of segments 13a and 13b function to constitute two legs of a unit segment.
Each of these unit segments are divided, at least at this portion from their bases up to the upper portion 13d' into two portions, by a slit 14a, and the lower portions 13d and 13e are also divided into two portions by a slit 14b which is contiguous to the above-mentioned slit 14a and constitute a radially and outwardly extending flanges 13d and 13e, and thereby they constitute L shaped legs and feet as shown by FIG. 7B. The bottom portion 17 is fabricated separately of the segments 13, and the bottom portion 17 is inserted within the inner space defined by the bottom of the side wall 13b as a collected body of the unit segments through the insulating material 18 as shown in FIG. 7A.
When the second type cold crucible furnace is compared with the first type one, there is observed almost same the structural features with respect to the induction coil and its pipings for a cooling water explained in item b) and supply and control means to supply melting power and cooling water as explained in item c) are substantially the same, so further explanation will not be repeated.
The drawbacks of the first type conventional cold crucible melting furnace is, that the molten metal kept non-contacted with the side wall of the crucible is kept at molten state, however, at the bottom it has no magnetic flux at its portion other than its outside region, and in addition, these two portions are contacted with each other since they are not sustained by the magnetic flux at these portions.
Due to these reasons, at the part lower than the central portion of the molten metal raised upward like a dome, there is a solidified portion called skull 9 as shown in FIG. 6. which is attributable to degrade the melting efficiency.
Particularly, the portion where the skull contacts the interior part of the side wall of the crucible not only makes the heat conductivity loss larger, but also even the heat induction from the surface does not contribute to improve the efficiency of melting.
In the second type conventional cold crucible furnace as compared with the first type one, there are following differences.
When the side wall of the crucible is situated on the radial extension of the upper face of the crucible bottom, the passage of the magnetic flux at the bottom portion mainly passes its slit portions.
This is because the magnetic resistance passing through the portions L.sub.1 and L.sub.2 is larger than that passes from the slit as shown in FIG. 7B.
And yet as shown in FIGS. 7A and 7B, side wall of the crucible is formed to have an L shaped cross section, the coils so as to avoid this L shaped crucible wall, must be moved upwards, or the diameter of the coils must be enlarged, however, the enlargement of the coil is inevitable and greatly lowers its efficiency as a whole.
By taking either way of 1) or 2), it results in lowering the magnetic flux density of the system.